Snap action pressure switch



United States Patent 9 Claims ABSTRACT OF THE DISCLOSURE A pressure responsive snap action type switch utilizing a concave porous element with an electrically conductive surface coacting with a contoured diaphragm having an electrically conductive surface, and pressure responsive means for actuating the diaphragm operatively.

This application is a continuation of application Ser. No. 491,164, filed Sept. 29, 1965, now abandoned.

This invention relates to an improved pressure switch to connect or open electrical circuits as a function of sensed fluid pressure.

There is need for a pressure switch that is capable of being actuated by relatively small differential pressures at high levels of absolute pressure. There is also a need for a pressure switch that can tolerate large over-pressures without damage to the sensitive element. A switch meeting these requirements should desirably be of the snap-action type so as to provide a dead-band between the actuating pressure and de-energizing pressure and to prevent chattering.

Existing pressure switches utilize either flexible elements, such as diaphragms and bellows, or pistons to sense pressure. The sensitive element is spring loaded; and motion of the element in response to applied pressure actuates separate electrical contacts to provide the switching function. Flexible elements sufficiently sensitive to react to small pressure differences are too delicate to withstand large swings of pressure in excess of that required for the desired switch actuation. If the flexible elements are made strong enough to withstand high pressure levels they become too stiif to react sufficiently to small pressure differences. Piston elements can be lightly loaded so as to be sensitive to small pressures. The piston can be seated after its required travel and withstand high pressure levels. However, switches with pistons can only be utilized where leakage can be tolerated. If no leakage is permitted, seals, such as O-rings, must be used with the piston. The resulting friction, increasing with pressure level, destroys the sensitivity of the piston element.

One of the objects of this invention is to provide a pressure switch of high sensitivity.

Another object of this invention is to provide a pressure switch that can withstand high levels of differential pressure.

A third object of this invention is to provide a pressure switch that eliminates leakage of the fluid being sensed.

A further object of this invention is to provide a pressure switch with snap-action.

The invention is based on the use of a diaphragm as the combined pressure sensitive element and movable electrical contact of a pressure switch. The diaphragm is located between two porous elements with electrically conductive surfaces, forming a pressure seal between them. The opposite sides of the diaphragm are also conductive. When the differential pressure across the diaphragm is negative compared to a reference the diaphragm seats against the conductive surface of one porous element providing a closed contact between that surface and the corresponding conductive surface on the diaphragm. When the differential pressure is positive compared to a reference the diaphragm changes position, and the opposite conductive surface seats against and provides a contact with the conductive surface of the other porous element. All conductive surfaces are connected to external circuits through insulated leads. Pressure is applied to opposite sides of the diaphragm through the porous elements. The porous elements also provide continuous support for the diaphragm in the normal and actuated positions, protecting the diaphragm against over-pressures, which serve to improve the electrical contact between mating surfaces.

Snap-action can be provided by giving the diaphragm a contour in one direction. When the differential pressure exceeds the reference value the diaphragm will be forced to another stable position having a contour in the opposite direction. The conductive surfaces of the porous elements are appropriately contoured to mate with the contoured diaphragm. Snap-action is best if the diaphragm is made of a spring material. The effect is analogous to the buckling of an oil can surface.

Various further and more specific features and advantages of the invention are hereinafter described in connection with the accompanying drawings.

In the drawings,

FIG. 1 is a sectional elevation view of the improved pressure switch showing a preferred form of the invention,

FIG. 2 is a sectional elevation view of a pressure switch with a pressure reference adjustment,

FIG. 3 is a fragmentary view showing a variation of diaphragm construction.

Referring to FIG. 1, the pressure switch is contained within a housing 10 and a cover 12 for the housing. Diaphragm 14 is held between two porous elements, 16 and 18. The diaphragm is shown as a dished metallic member in its normal or unpressurized position. Porous element 16 has an upper surface 20, which is contoured to mate with the bottom surface of the diaphragm when the differential pressure across the diaphragm is negative compared to a reference value; and the diaphragm is in the normal position. Surface 20 is in intimate contact with the diaphragm, and supports it over the entire surface area. Porous element 18 has a lower surface 22, opposite to surface 20; and it is contoured to rnate with the top surface of the diaphragm when the differential pressure across the diaphragm is positive compared to the reference value; and the diaphragm is in the actuated or pressurized position. The lower surface of the diaphragm in its actuated position is shown by dashed line 23. Surface 22 is in intimate contact with the diaphragm and supports it over the entire surface area when the diaphragm is in the actuated position. Elements 16 and 18 are shown made of porous metal so that they are electrically conductive and serve as two electrodes of the pressure switch, the diaphragm itself acting as a third electrode.

The diaphragm is also held on its outer surface between supports 24 and 26, made of ceramic or other material that is not electrically conductive. Supports 24 and 26 retain porous elements 16 and 28 and act as insulators for them as well as for the diaphragm.

Insulator 2-8 in housing 10 supports porous element 16. Insulator 30 in cover 12 supports porous element 18. The insulators also transmit pressure loads to the housing and cover. The insulators are made of alumina or other non conductive material with high compressive strength.

Porous electrode element 16 can be electrically connected to an external circuit through terminal screw 32, with which it is in contact. Porous electrode element 18 can be electrically connected to an external circuit through terminal screw 34, with which it is in contact. Terminal 32 is screwed into threaded hole 36 of insulator 28. Terminal 34 is screwed into threaded hole 38 of insulator 30. The diaphragm can be electrically connected to an external circuit through terminal 40'.

The diaphragm is clamped between the housing and the cover along its outer flange 42. Flange 42 is electrically insulated on both sides to prevent conduction between the diaphragm, and both the housing and the cover. The diaphragm is also held along the circular area at which it bends during actuation by two inserts 44 and 46, made 'of rubber or other resilient material. The inserts prevent cracking due to sharp corners at the bending ring; and they also insulate the diaphragm from the porous electrode elements at the bending ring to assure that the diaphragm is in contact with only one of the two electrodes, depending on its position, normal or actuated.

Ports 48 in housing and 50 in cover 12 are used to connect the pressure switch to external sources of fluid pressure. Pressure is applied to the bottom surface of the diaphragm through port 48, annular channel 52 in support 24, and porous element 16. Pressure is applied to the upper surface of the diaphragm through port 50, annular channel 54 in support 26, and porous element 18.

The housing and cover are held together by a number of bolts 56. O-rings 58 in housing 10 and O-ring 60 in cover 12 prevent external leakage.

A certain force must be applied across the diaphragm to cause it to buckle from its normal position to its actuated position. This force level, which is a function of the dimensions and composition of the diaphragm, determines the reference value of differential pressure, which must be exceeded to actuate the pressure switch. In operation, the diaphragm will be in its normal position when the difference between the pressure applied to port 48 and the pressure applied to port 50 is negative with respect to the reference value. When the pressure difference is positive with respect to the reference value, that is when the pressure applied to port 48 exceeds that applied to port 50 by a certain amount, the diaphragm buckles in cavity 62 toits actuated position. In its normal position the pressure switch provides a closed contact to an external circuit through lead 64, terminal 32, porous electrode element 16, diaphragm 14, terminal 40 and lead 68. The circuit between leads 66 and 68 is open. In its actuated position the pressure closes the circuit between leads 66 and 68 through terminal 34, porous electrode element 18, diaphragm 14 and terminal 40.

The embodiment shown in FIGURE 2 is a variation in which means are provided to adjust the reference value of differential pressure. The pressure switch shown includes a housing 70 with a cover 72. The contoured diaphragm 74 consists of a non-conductive center layer 76 and conductive surfaces 78 and 8t). :Porous elements '82 and 84, supports 86 and :88 and insulators 90 and 92 have the same functions as corresponding elements in FIG. 1. In place of screw terminals, leads 94 and 96 pass through insulators 98 and 100 to connect the porous elements to external circuits. Conductive surfaces 78 and 80 on the diaphragm are connected to external circuits through leads 102 and 104, respectively.

The pressure reference-adjusting means consists of a pin 106, which passes through a close fitting hole in porous element 84 and bears against surface 80 of the diaphragm. The pin is loaded by coil spring 108, whose bias force is varied by turning adjustment screw 110. The travel of the pin is limited so that when the diaphragm is actuated the spring end of the pin is seated against cover 72 and the bearing end is flush with the contact surface of porous element 84.

Flange 112 of the diaphragm is again insulated to electrically isolate conductive surfaces 78 and 80. Resilient inserts 114 and 116 serve the same function as inserts 44 and 46 in FIG. 1.

Fluid pressure is applied to the diaphragm through ports 118 and 120, annular channels 122 and 124 and porous elements '82 and 84.

Housing and cover 72 are fastened with bolts 126 and sealed by O-rings 128 and 130.

The operation of the pressure switch shown in FIG. 2 is similar to that of the embodiment of FIG. 1. When the pressure difference becomes positive compared to the reference the diaphragm moves through cavity 132 until surface seats against porous element 84. The clearance of pin 106 in its hole is made very small; and the end of the pin is made flush with the contact surface to close tolerances so that no cavities are provided for extrusion of the diaphragm under high pressures. The free length of the coil spring is large compared to the travel of the pin so as to minimize variation of the force reference as the diaphragm travels to its actuated position. Buckling of the diaphragm can be represented as a negative spring characteristic, which is reduced by the positive characteristic of spring 108. The latter rate is kept sufficiently low that the net effect is still that of a negative spring.

It is evident that many variations can be made without affecting the essential structure and function of the pressure switch. For example, the diaphragm can be convoluted to increase flexibility and minimize depth. The supports and insulators can be varied in shape or eliminated. The porous elements can be made of non-conductive porous material with a thin conductive porous layer; or they can be made of conductive porous metal covered with an insulating coating, such as Teflon.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In an electrical switch of the pressure responsive type, a housing, a porous element with an electrically conductive surface mounted in the housing, a diaphragm mounted in the housing having an electrically conductive surface contactable with the conductive surface of the porous element, and pressure responsive means for actuating the diaphragm operatively connected to the porous element.

2. A pressure switch of the character claimed in claim 1 including means to adjust the value of pressure at which the diaphragm is actuated.

3. A pressure switch of the character claimed in claim 1 in which the diaphragm is contoured in one direction when the pressure is negative with respect to a reference and in an opposite direction when the pressure is positive with respect to the reference and the conductive surface of the porous element is contoured to mate with the diaphragm when the pressure is negative with respect to the reference.

4. A pressure switch of the character claimed in claim 1 in which the diaphragm is contoured in one direction when the pressure is negative with respect to a reference and in an opposite direction when the pressure is positive with respect to the reference and the conductive surface of the porous element is contoured to mate with the diaphragm when the pressure is positive with respect to the reference.

5. A pressure switch of the character claimed in claim 2 in which the pressure reference adjustment includes a spring loaded pin bearing on the diaphragm and means to vary the pre-load of the spring in order to adjust the pressure reference.

6. A pressure switch of the character claimed in claim 1 in which the porous element is metallic and insulated from the housing.

7. A pressure switch of the character claimed in claim 1 including a second porous element with a conductive surface opposite to the conductive surface of the first 5 6 porous element and a diaphragm with a second conduc- 1,692,513 11/1928 Newell. tive surface contactable with the conductive surface of 2,794,033 5 /1957 Ostby the second porous element.

8. A pressure switch of the character claimed in claim FOREIGN PATENTS 7 in which the diaphragm is metallic and the conductive surfaces are electrically common.

9. A pressure switch of the character claimed in claim 7 in which the conductive surfaces of the diaphragm are 5 221,504 5/1925 Great Britain.

insulated from each other H. BURKS, Assistant Examiner.

References Cited 10 C1. XR. UNITED STATES PATENTS 20083 1,183,486 5/1916 Pardue.

ROBERT K. SCHAEFER, Primary Examiner. 

